Process for preparing nitriles



United States Patent 9 PROCESS FOR PREPARING NITRILES Werner Mnench andEnzo Ruoti, Cesano Maderno, and Giuliana Silvestri, Milan, Italy,assignors, by mesne assignments, to Snia Viscosa Societa NazionaleIndustria Applicazioni Viscosa S.p.A., Milan, Italy, a company of ItalyNo Drawing. Application March 16, 1956 Serial No. 571,897

Claims priority, application Italy March 18, 1955 6 Claims. (Cl. 260465)It has been well-known for a long time (B 17 (1884), 73, 2332, 2337,2343; B 18 (1885), 138, 1002), that the decomposition of formylamines byheat forms nitriles, but the yield of nitriles in said decomposition byheat did not surpass 20% of the formylamines employed.

Later on it was found (Mailhe Or. 176, 689 (1923), that on passingformaniline vapour over A1 at 400 C., a small amount of benzonitrile isformed in addition to carbon monoxide and aniline.

These observations were taken up in German patent specification No.482,943 (1929), in which it is shown that by passing formic salts ofprimary amines or formylamides over porous contact substances, therespective nitriles are generally obtained.

The reaction that takes place is probably as follows: at first theisonitriles are formed from the formylamines, and from the isonitrilesthere are formed the nitriles by transposition at very high temperature.This may be represented by the following reaction scheme:

Benzonitrile is obtained by a similar reaction, here too through theintermediate formation of isonitrile by heating phenyl isosulfocyanateWith powdery copper (B 6, 212 (1873)). German patent specification No.259,364 describes, and is based on, this observation.

While the second mentioned process has but slight industrial importance,owing to the comparatively high cost of raw materials, the firstmentioned process might achieve fundamental importance, sinceformylamines can be obtained in a comparatively easy way from amines andcarbon monoxide, the cost of which is not high.

According to patent specification No. 482,943, as described in theexamples, vapours of formic salts of amines, of formylamines or ofmixtures of amines with esters of formic acid are passed over contactsubstances; now, it.

is hardly possible to prepare vapours of formic salts of amines as suchand, therefore, to convey them through pipes; at any rate, in theexamples of specification 482,943 it is not even said in what mannersuch vapours are produced and transported. It is assumed that probablythey reach the catalyst in a liquid state and evaporate when touchingthe contact mass. operating in this way or in a similar manner, it isnot possible to avoid decompositions. Consequently the process accordingto said specification gives low yields in the majority of cases.specification may serve as a basis for computing the yields only in somecases, since no distinction is made between conversion after one passageand total yield.

It is evident that when Also the examples in said In addition to thefact that the yields are comparatively low, operation in accordance withthe cited patent specification causes a quick separation of resinousproducts, so that after even a few hours the contacts are useless.

Now it has been found that these drawbacks can be avoided and that it ispossible to increase yields and conversions to a relevant extent, bypassing the vapours of the formyl compounds over the catalyst only in ahighly diluted condition. This can be obtained by passing through themolten formyl compounds heated to elevated temperature, a stream of aninert gas which becomes saturated with the vapours of said formylcompounds by evaporation of the latter, and carries said compounds sodiluted, to rearrangement through the catalyst. As an inert gas it ispossible, to use for instance, nitrogen, carbon monoxide or hydrogen. Astill better result has been attained by operating under vacuum insteadof with an inert gas. The formyl compounds are evaporated by the use ofvacuum, e.g. of 20-40 mm. Hg, and the vapours so diluted are made toreact directly through the catalyst,

without using further diluents. In both cases, the evaporatingtemperatures of the formyl derivatives should be as high as possible toobtain a velocity of evaporation high enough to allow transportation ofvapours but, on the other hand, said temperatures should remain belowthe decomposition point of the corresponding formyl amines, otherwisethe yields will decrease. Generally speaking, the most convenienttemperatures are those corresponding to the boiling point of the formylderivative at a vacuum of 20-30 mm.

The improvement attained in the stability and permanence of the catalystand above all in yield and in conversion, especially if vacuum is used,is so unexpectedly marked that it cannot be explained in any obviousmanner. It is true that formyl amides are often sensitive to hightemperatures and decompose easily with the separation of CO. This mayexplain why it is possible to increase the yields if evaporation iscarried out with greater care. However, the substantial improvementsaiforded by the new process are not likely to be due only to thisfactor.

As already said above, it is assumed that an isonitrile is obtained asan intermediate product in the formation of nitriles, which isonitriletransforms into a nitrile at the elevated temperature of the catalystchamber. Better to explain the unexpected improvement due to the newprocess, yields of 98100% are obtained, calculated on the amineemployed, and conversions up to -80% after;

one passage only.

A further and essential advantage of this novel process is that thecatalyst remains active for some weeks; conversions diminish slightlyonly after much prolonged use, but even then the yields are not lowered.When after some weeks of operation the conversion with one passage hasat last diminished by 5 to 8%, it suffices merely to treat the catalystwith air to render it quite effective again.

The present process can be employed with particular success for thepreparation of those nitriles which carry on their nucleus a furtherhydrocarbon group. For in stance, it serves to prepare p-tolunitrilefrom p-toluidine (p-tolunitrile being a starting material for preparingterephthalic acid and synthetic resins); and cyancurnene (which too is astarting material for synthetic resins) from p-aminocumene.

Broadly speaking, the operational steps of the new process are asfollows:

The formyl amine to be reacted is melted and heated to boiling under avacuum of 20 mm. of Hg in a vessel that can be heated up to ZOO-250 C.in an oil bath. The vessel is provided with a discharge pipe and athermometer. The vapours produced pass through the discharge tube into apipe heated to about 500 C., which is packed either with silica gel, orwith carbon (charcoal), aluminum oxide, boron phosphate or another knowncatalyst; the vapours pass through this tube, whereafter the products ofreaction are condensed with the usual means.

' If the operation is carried out without using any vacuum, the vesselis equipped with a pipe for the introduction of a gas, which extends tothe bottom of the vessel and is connected with a source of nitrogen,carbon monoxide or hydrogen. The pipe may be provided with devices forthe better distribution of the gas stream within the liquid. Pre-heatingof the gas is not required, but may be provided. The gases saturatedwith the vapours of formylamine pass through the apparatus in the manner hereinbefore described in setting forth the first embodiment of theprocess.

For the treatment of the reaction products, the usual method may beemployed. Usually the reaction product is composed of free amine andnitrile, but it may contain a very small portion of unreacted formylderivative. It may be treated e.g. with diluted mineral acids; the aminedissolves and the nitrile separates as an insoluble layer; it can beseparated and finally purified by distillation.

The aqueous solution containing the amine is rendered alkaline, and thiscauses the separation of said amine, which can be recovered.

The formyl derivatives that may be present in the product if thestarting compounds contain a side chain, generally dissolve in thenitrile from which they can be separated by distillation. In the case ofaniline, the greater part of the formyl derivative is found in theaniline layer.

In lieu of a diluted mineral acid, formic acid may be used and so,following conventional methods, the formyl derivative may be prepareddirectly from the formic salt of the amine.

The following examples better illustrate the process:

Example 1 The apparatus consists of an iron vessel of 3 litres capacityplaced in an oil bath adapted to be heated and on which there is mounteda vacuum-proof cover or lid. The lid carries a joint with a cock for theintroduction of the starting material, a joint with a thermometer and ajoint for the connection of the catalyst tube. The catalyst tube is asimple iron tube with electric heating, having an internal diameter of30 mm., on the bottom of which there is applied a coarse mesh sieve,upon which the catalyst is placed in a layer of about 100 cm. A silicagel of 4-5 mm. grain size is used as a catalyst. The temperature in thecatalyst tube is kept at 490500 C. by means of a thermometer providedwith an adjusting device. At the upper end of the catalyst pipe there isapplied a downward condenser ending in a receiving vessel connected withthe vacuum pump.

To carry out the experiment, 1730 g. of molten formanilide areintroduced into the vessel, the vacuum pump is started and the oil bathis heated to 245-255 C. with a vacuum of 20 mm. and an internaltemperature of about 185 C. there are evaporated about 200 g./hour offormanilide. The vapours pass through the catalyst chamber kept at 500C. and are thereafter condensed in the cooler and collected in thereceiving vessel. When Alt all the formanilide is evaporated, theexperiment is broken off and the distillate is worked up. It is stirredwith diluted hydrochloric acid in such an amount as to render thesolution acid, with a pH equal to 1, and the layers formed areseparated. The aqueous portion is stirred again with ether and the ethersolution is joined to the oil, previously separated. After evaporatingthe ether and distilling the residue, 810 g. of benzonitrile and 45 g.of unreacted formanilide are obtained, which come sponds to a 58.4%conversion of formanilide into benzonitrile. The aqueous portion is madealkaline by means of sodium hydroxide and the aniline separated isdistilled; 482 g. are obtained. The total yield, taking into account theaniline recovered, is 99%, calculated on the aniline employed.

When working without vacuum according to German Patent 482,943 (Example3), from aniline and ethyl ester of formic acid there is obtained abenzonitrile yield of only 25% of the theoretical amount.

Example 2 332 g. of formyl cumidine, obtained by formylation of a crudemixture of (about 15%) oand (about p-cumidine (isopropylaniline), areevaporated under a vacuum of 30 mm. Hg and a temperature of 220230 C.;the vapours are passed over the catalyst heated to 500 C., as describedin Example 1, and the condensate is treated as before: 166 g. ofisopropylbenzonitrile and 77 g. of cumidine are obtained. In this casethe conversion into isopropylbenzonitrile is 56.1% and the aminerecovered is 28.1%, both figures calculated on isopropylformamide. Theyield is only 85%, owing to the impurity of the starting materialemployed. Hence the ratio between the amounts of amine and of nitrile,is almost exactly 1:2. Comparative experiments carried out without usingvacuum, gave about the same yields of 85%, but the average ratio betweenamine and nitrile amounts was only 1:1.2.

Example 3 965 g. of p-methyl formanilide are evaporated as in Example 1with a vacuum of 20 mm. Hg, an internal temperature of 185-190 C. and ata speed of about 200 g./hour and the vapours are passed over thecatalyst heated to 500 C. From the condensate, 240 g. of ptoluidine and5 64 g. of p-tolunitrile (p-methylbenzonitrile) are obtained.

Taking into account the amount of substance recovered, the yield is98.3% of theory. The conversion of formyl toluidine into tolunitrile is67.5%. Comparative experiments without the application of a vacuum givea yield of 89% of the theoretical amount and the conversion ofp-formyltoluidine into p-tolunitrile is 35% of the theoretical amount.

Example 4 300 g. of p-methyl formanilide are heated at atmosphericpressure to 210 C. and a nitrogen stream is passed through the moltenmass. The formyl amide vapours entrained by the nitrogen, pass over thecatalyst heated to 500 C. There are obtained 280 g. of a condensate,from which g. of p-methyl benzonitn'le are obtained. Moreover, 85 g. ofp-toluidine are recovered. Consequently this process yields 98.9% of thetheoretical amount of p-methyl benzonitrile. Conversion is 63.5%.

We claim:

1. The method of preparing monocarbocyclic aryl nitriles of the formulaRCN, where R is monocarbocyclic aryl, comprising passing vapors of amonocarbocyclic formyl amine of the formula RNHCOH, wherein R ismonocarbocyclic aryl, over a dehydration catalyst under vacuum in therange of 20-40 mm. Hg at elevated temperatures.

2. The method as defined in claim 1 wherein the vapors of the saidformyl amine are produced by distilling under vacuum in the range of20-40 mm. Hg to form vapors thereof prior to contact with the catalyst.

3. The method of preparing monocarbocyclic aryl nitriles of the formulaRCN, where R is monocarbocyclic 5 aryl, comprising melting a monocyclicformyl amine of the formula RNHCOH, wherein R is monocarbocyclic aryl,at atmospheric pressure, passing a stream of inert gas through themolten mass, and conducting vapors of the formyl amine in the inert gasover a dehydration 10 catalyst.

References Cited in the file of this patent FOREIGN PATENTS 482,943Germany Sept. 25, 1929

1. THE METHOD OF PREPARING MONOCARBOCYCLIC ARYL NITRILES OF THE FORMULARCN, WHERE R IS MONOCARBOCYCYLIC ARYL, COMPRISING PASSING VAPORS OF AMONOCARBOCYCLIC FORMYL AMINE OF THE FORMULA RNHCOH, WHEREIN R ISMONOCARBOCYCLIC ARYL, OVER A DEHYDRATION CATALYST UNDER VACUUM IN THERANGE OF 20-40 MM, HG AT ELEVATED TEMPERATURE.